Process of making a photosensitive semi-aqueous developable ceramic coating composition

ABSTRACT

A photosensitive ceramic coating composition which is fireable in a substantially nonoxidizing atmosphere comprising an admixture of: 
     (a) finely divided particles of ceramic solids, 
     (b) finely divided particles of an inorganic binder dispersed in an organic medium comprising 
     (c) an organic polymeric binder, and 
     (d) a photoinitiation system, dissolved in 
     (e) photohardenable monomer, and 
     (f) an organic medium 
     wherein the improvement comprises an organic polymeric binder containing a copolymer or interpolymer of a C 1  -C 10  alkyl acrylate, C 1  -C 10  methacrylate, styrene, and an ethylenically unsaturated carboxylic acid, wherein a moiety in the binder derived from the unsaturated carboxylic acid comprises from 5 to less than 15 weight percent of the polymer and wherein the binder has a molecular weight not greater than 100,000 and wherein the composition upon imagewise exposure to actinic radiation is developable in an organic solvent-water mixture.

This is a division of application Ser. No. 200,386, filed May 31, 1988now U.S. Pat. No. 4,908,296.

BACKGROUND OF THE INVENTION

The present invention is directed to an improved photosensitive ceramiccoating composition and more particularly a coating composition whichfunctions as a precursor to a fired ceramic which has the ability toserve as a dielectric material particularly useful in formation ofmultilayer thick film circuits.

The present invention is directed to an improved photosensitive ceramiccoating composition which can be fired in a substantially nonoxidizingatmosphere disclosed in Dueber et al. U.S. Pat. No. 4,613 560 issuedSept. 23, 1986. This patent discloses a coating composition whichcomprises an admixture of:

(a) finely divided particles of ceramic solids having a surfacearea-to-weight ratio of no more than 10 m² /g and at least 75 wt. % ofthe particles having a size of 1-10 μm.

(b) finely divided particles of an inorganic binder having a surfacearea-to-weight ratio of no more than 10 m² /g and at least 95 wt. %, ofthe particles having a size of 1-10 μm, the weight ratio of (b) to (a)being 0.6-2, dispersed in an organic medium comprising.

(c) an organic polymeric binder selected from the group consisting of(1) homopolymer and copolymers of C₁₋₁₀ alkyl acrylates, C₁₋₁₀ alkylmethacrylates, alpha-methylstyrene and 0-2 wt. % ethylenicallyunsaturated carboxylic acid, amine or silane-containing compounds, (2)homopolymers and copolymers of C₁₋₁₀ alkyl mono-olefins, (3)homopolymers and copolymers of Cl₁₋₄ alkylene oxide and mixturesthereof, the binder comprising 5-25 wt. % basis total inorqanic solids,and

(d) a photoinitiation system, dissolved in

(e) photohardenable monomer and

(f) volatile nonaqueous organic solvent. As set forth in this patentprocessing involves the coating composition being (1) laminated to aceramic substrate, (2) exposed imagewise to actinic radiation to effecthardening the exposed areas of the film, (3) solvent developed to removeunexposed areas of the film, and (4) fired in a substantiallynonoxidizing atmosphere to effect volatilization of the organic mediumand sintering of the inorganic binder and ceramic solids.

As further background to the present invention multilayer thick filmcircuits have been used for many years to increase circuit functionalityper unit of area. Moreover, recent advances in circuit technology haveplaced new demands on dielectric materials for this use. Heretofore,most of the dielectric materials used in multiple circuits have beenconventional thick film dielectric compositions. These are comprised offinely divided particles of dielectric solids and inorganic bindersdispersed in an inert organic medium. Such thick film materials areusually applied by screen printing, though they may be applied by othermeans as well.

Thick film materials of this type are very important and will continueto be so. However, when applying these thick film materials by screenprinting, it is difficult to obtain fine line and space resolution. Itis essential that all the screen printing variables such as screenquality. sgueegee hardness, print speed, dispersion properties, etc., bemost carefully controlled and constantly monitored to obtain goodproduct yields. Similar problems exist, of course, with the use of thickfilm conductor and resistor materials.

One approach to this problem is (1) to apply a layer of the dielectricmaterial to a substrate by means of dispersion in a photosensitivemedium, (2) to expose the layer imagewise to actinic radiation, (3) tosolvent develop the pattern to remove unexposed portions of the layer,and (4) to fire the remaining exposed portions of the pattern to removeall remaining organic materials and to sinter the inorganic materials.

A disadvantage of prior art compositions and particularly compositionsdisclosed in U.S. Pat. No. 4,613,560 is an organic solvent is necessaryto develop such material after imagewise exposure to actinic radiation,i.e., an organic solvent removes areas of the composition which have notbeen exposed to actinic radiation without removal of areas which havebeen exposed.

SUMMARY OF THE INVENTION

The present invention is directed to a photosensitive ceramic coatingcomposition which is fireable in a substantially nonoxidizing atmospherecomprising an admixture of:

(a) finely divided particles of ceramic solids having a surfacearea-to-weight ratio of no more than 10 m² /g and at least 80 wt. % ofthe particles having a size of 1-10 μm, and

(b) finely divided particles of an inorganic binder having a glasstransition temperature in the range from of 550° to 825° C., a surfacearea-to-weight ratio of no more than 10 m² /g and at least 90 wt. % ofthe particles having a size of 1-10 μm, the weight ratio of (b) to (a)being in a range from 0.6 to 2, dispersed in an organic mediumcomprising

(c) an organic polymeric binder, and

(d) a photoinitiation system, dissolved in

(e) photohardenable monomer, and

(f) an organic medium

wherein the improvement comprises an organic polymeric binder containinga copolymer or interpolymer of a C₁ -C₁₀ alkyl acrylate or C₁ -C₁₀methacrylate, and an ethylenically unsaturated carboxylic acid, whereina moiety in the binder derived from the unsaturated carboxylic acidcomprises from 5 to less than 15 weight percent of the polymer andwherein the binder has a molecular weight not greater than 100,000 andwherein the composition upon imagewise exposure to actinic radiation isdevelopable in an aqueous solution containing by weight 0.62 percentsodium borate and 8.7 percent butyl cellosolve.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to an improved photosensitive ceramiccoating disclosed in Dueber et al, U.S. Pat. No. 4,613,560. Sincecomponents of this patent can be employed in the present invention asmodified herein, this patent is incorporated by reference with portionsof directly reproduced herein. Also the same processing steps can beemployed with compositions of the present invention as in U.S. Pat. No.4,613,560 except a different developer is employed to remove areas ofthe composition which have not been exposed to actinic radiation withoutremoval of areas which have been exposed to such radiation.

Ceramic Solids

The invention is applicable to virtually any high melting inorganicsolid material. However, it is particularly suitable for makingdispersions of dielectric solids such as alumina, titanates. zirconatesand stannates. It is also applicable to precursors of such materials,i.e., solid materials which upon firing are converted to dielectricsolids, and to mixtures of any of these.

Among the many dielectric solids which are likely to be used in theinvention are BaTiO₃, CaTi₃, SrTiO₃, pbTiO₃, CaZrO₃, BaZrO₃, CaSnO₃,BaSnO₃, and Al₂ O₃. As will be apparent to those skilled in the ceramicarts, the exact chemical composition of the ceramic solids to be used inthe composition of the invention is not ordinarily critical in therheological sense. It is also preferred that the ceramic solids not haveswelling characteristics in the organic dispersion since the rheologicalproperties of the dispersion may be substantially changed thereby.

It has been found that the dispersion of the invention must contain nosignificant amount of solids having a particle size of less than 0.2 μmin order to obtain adequately complete burnout of the organic mediumwhen the films or layers thereof are fired to remove the organic mediumand to effect sintering of the inorganic binder and the ceramic solids.However, none of the ceramic solids ordinarily will exceed 20 μm and,furthermore, at least 80 wt. % of the ceramic solids must have a size of1-10 μm. When the dispersions are used to make thick film pastes, whichare usually applied by screen printing, the maximum particle size mustnot exceed the thickness of the screen, and when the dispersion is usedto make dry photosensitive film, the maximum particle size must notexceed the thickness of the film. It is preferred that at least 80 wt. %of the ceramic solids fall within the 1-10 μm range.

In addition, it is preferred that surface area/weight ratio of theceramic particles not exceed 10 m² /g for the reason that such particlestend to affect adversely the sintering characteristics of theaccompanying inorganic binder. It is still further preferred that thesurface area/weight ratio not exceed 5 m² /g. Ceramic particles having asurface area/weight ratio of 1-5 have been found to be quitesatisfactory.

A preferred particle size for d50, i.e., a point at which the weight ofsmaller particles equal the weight of larger particles, is in a rangefrom 2.30-2.70 microns. This size range is preferred to achieve ablister free surface while maintaining a hermetic structure. Thisparticle size range is similar to a preferred range for inorganic binderof glass frit, namely a range from 2.30-2.70 microns d50).

A preferred material is alumina with a preferred procedure is to passthe milled water alumina slurry through a fine mesh screen (ex 400 mesh)to remove large particles and also through a magnetic separator. Themagnetic separator removes all magnetic conductive materials whichotherwise would increase the conductivity of the final product.

Of critical importance is to achieve a ceramic solid composition whichis agglomerate free. Normally after milling a ceramic, to achievedesired particle size distribution, the water or mixing solvent isremoved by either vacuum assisted heating or heating in an air stream.This technique results in some of the ceramic particulates becomingagglomerated, resulting, after firing, in a part with large particulatesclusters and/or blisters.

This particularly serious disadvantage is eliminated in a preferredprocedure by freeze drying the milled alumina. The very mild conditionsof freeze drying eliminates the potential for agglomeration whichnormally results when powders are heated. A similar technique isapplicable for different ceramic solids.

Inorganic Binder

The glass frit used in the present invention aids in sintering theinorganic crystalline particulates and may be of any well knowncomposition which has a melting temperature below that of the ceramicsolids. Nevertheless, in order to get adequate hermeticity of thedevices, it is preferred that the glass transition temperature (T_(g))of the inorganic binder be 550°-825° C. and still more preferably575°-750° C. If melting takes place below 550° C. organic material willlikely be encapsulated and blisters will tend to form in the dielectriclayer as the organics decompose, On the other hand, a glass transitiontemperature above 825° C. will tend to produce a porous dielectric whensintering temperatures compatible with copper metallizations, e.g., 900°C. are used.

The glass frits most preferably used are the borosilicate frits, such aslead borosilicate frit, bismuth, cadmium, barium, calcium or otheralkaline earth borosilicate frits. The preparation of such glass fritsis well known and consists, for example, in melting together theconstituents of the glass in the form of the oxides of the constituentsand pouring such molten composition into water to form the frit. Thebatch ingredients may, of course, be any compound that will yield thedesired oxides under the usual conditions of frit production. Forexample, boric oxide will be obtained from boric acid, silicon dioxidewill be produced from flint, barium oxide will be produced from bariumcarbonate, etc. The glass is preferably milled in a vibratory (SwecoCo.) mill with water to reduce the particle size of the frit and toobtain a frit of substantially uniform size.

Thereafter the frit is preferably processed in a similar manner as theceramic solids. The frit is passed through a fine mesh screen to removelarge particles since the solid composition should be agglomerate free.The inorganic binder like the ceramic solids should have a surface toweight ratio of no more than 10 m² /g and at least 90 wt. % of theparticles preferably have a particle size of 1-10 μm.

It is preferred that the d50 of the inorganic binder, which is definedas equal parts by weight of both larger and smaller particles, be equalto or less than that of the ceramic solids, for given particle sizeceramic solids, the inorganic binder/ceramic solids ratio required toachieve hermeticity will decrease as the inorganic binder sizedecreases. With a given ceramic solids-inorganic binder system, if theratio of inorganic binder to ceramic solids is significantly higher thanthat required to achieve hermetically the dielectric layer tends to formblisters on firing. If the ratio is significantly lower, the fireddielectric will be porous and therefore nonhermetic.

Within the above-described particle size and surface area limits, it isnevertheless preferred that the inorganic binder particles be 0.5-6 μm.The reason for this is that smaller particles having a high surface areatend to adsorb the organic materials and thus impede cleandecomposition. On the other hand, larger size particles tend to havepoorer sintering characteristics. It is preferred that the ratio ofinorganic binder to ceramic solids be 0.6-2.

Photoinitiation System

Suitable photoinitiation systems are those which are thermally inactivebut which generate free radicals upon exposure to actinic light at orbelow 185° C. These include the substituted or unsubstituted polynuclearquinones which are compounds having two intracyclic carbon atoms in aconjugated carbocyclic ring system. e.g., 9,10-anthraquinone,2-methylanthraquinone. 2-ethylanthraquinone 2-tert-butylanthraquinone,octamethylanthraquinone, 1.4-naphthoquinone, 9,10-phenanthrenequinone,benz(a)anthracene-7,12-dione. 2,3-naphthacene-5,12-dione,2-methyl-1,4-naphthoquinone, 1,4-dimethyl-anthraquinone.2,3-dimethylanthraquinone, 2-phenylanthraquinone.2,3-diphenylanthraquinone, retenequinone.7,8,9,10-tetrahydronaphthacene-5,12-dione, and1,2,3,4-tetrahydrobenz(a)anthracene-7,12-dione. Other photoinitiatorswhich are also useful, even though some may be thermally active attemperatures as low as 85° C. are described in U.S. Pat. No. 2,760,863and include vicinal ketaldonyl alcohols such as benzoin pivaloin acyloinethers, e.g. benzoin methyl and ethyl ethers; α-hydrocarbon-substitutedaromatic acyloins, including α-methylbenzoin α-allylbenzoin andα-phenylbenzoin, photoreducible dyes and reducing agents disclosed inU.S. Pat. Nos. 2,850,445, 2,875,047, 3,097,096, 3,074,974, 3,097,097,and 3,145,104. as well as dyes of the phenazine oxazine, and quinoneclasses. Michler's ketone, benzophenone, 2,4,5-triphenylimidazolyldimers with hydrogen donors including leuco dyes and mixtures thereof asdescribed in U.S. Pat. Nos. 3,427,161, 3,479,185 and 3,549,367 can beused as initiators. Also useful with photoinitiators and photoinhibitorsare sensitizers disclosed in U.S. Pat. No. 4,162,162. The photoinitiatoror photoinitiator system is present in 0.05 to 10% by weight based onthe total weight of the dry photopolymerizable layer.

Photohardenable Monomer

The photohardenable monomer component of the invention is comprised ofat least one addition polymerizable ethylenically unsaturated compoundhaving at least one polymerizable ethylenic group. Such compounds arecapable of forming a high polymer by free radical initiated, chainpropagating addition polymerization. The monomeric compounds arenongaseous. i.e., they have a normal boiling point above 100° C. and aplasticizing action on the organic polymeric binder.

Suitable monomers which can be used alone or in combination with othermonomers include t-butyl acrylate and methacrylate, 1,5-pentanedioldiacrylate and dimethacrylate, N,N-diethylaminoethyl acrylate andmethacrylate ethylene glycol diacrylate and dimethacrylate,1,4-butanediol diacrylate and dimethacrylate, diethylene glycoldiacrylate and dimethacrylate, hexamethylene glycol diacrylate anddimethacrylate, 1,3-propanediol diacrylate and dimethacrylate,decamethylene glycol diacrylate and dimethacrylate, 1,4-cylohexanedioldiacrylate and dimethacrylate, 2,2-dimethylolpropane diacrylate anddimethacrylate, glycerol diacrylate and dimethacrylate, tripropyleneglycol diacrylate and dimethacrylaIe, glycerol triacrylate andtrimethacrylate, trimethylolpropane triacrylate and trimethacrylate,pentaerythritol triacrylate and trimethacrylate, polyoxyethylatedtrimethylolpropane triacrylate and trimethacrylate and similar compoundsas disclosed in U.S. Pat. No. 3,380,831, 2,2-di(p-hydroxyphenyl)-propanediacrylate, pentaerythritol tetraacrylate and tetramethacrylate,2,2-di-(p-hydroxyphenyl)-propane dimethacrylate, triethylene glycoldiacrylate, polyoxyethyl-2,2-di-(p-hydroxyphenyl)propane dimethacrylate,di-(3-methacryloxy-2-hydroxypropyl) ether of bisphenol-A,di-(2-methacryloxyethyl) ether of bisphenol-A,di-(3-acryloxy-2-hydroxypropyl) ether of bisphenol-A,di-(2-acryloxyethyl) ether of bisphenol-A,di-(3-methacryloxy-2-hydroxypropyl) ether of 1,4-butanediol, triethyleneglycol dimethacrylate, polyoxypropyltrimethylol propane triacrylate,butylene glycol diacrylate and dimethacrylate 1,2,4-butanetrioltriacrylate and trimethacrylate, 2,2,4trimethyl-1,3-pentanedioldiacrylate and dimethacrylate, 1-phenyl ethylene-1,2-dimethacrylate,diallyl fumarate, styrene, 1,4-benzenediol dimethacrylate1,4-diisopropenyl benzene, and 1,3,5-triisopropenyl benzene. Also usefulare ethylenically unsaturated compounds having a molecular weight of atleast 300, e.g., alkylene or a polyalkylene glycol diacrylate preparedfrom an alkylene glycol of 2 to 15 carbons or a polyalkylene etherglycol of 1 to 10 ether linkages, and those disclosed in U.S. Pat. No.2,927,022, e.g., those having a plurality of addition polymerizableethylenic linkages particularly when present as terminal linkages,preferred monomers are polyoxyethylated trimethylolpropane triacrylate.ethylated pentaerythritol triacrylate, dipentaerythritolmonohydroxypentaacrylate and 1.10-decanediol dimethylacrylate. Theunsaturated monomeric component is present in an amount of 5 to 45% byweight based on the total weight of the dry photopolymerizable layer.

ORGANIC MEDIUM

The main purpose of the organic medium is to serve as a vehicle fordispersion of the finely divided solids of the composition in such formthat can readily be applied to a ceramic or other substrate. Thus, theorganic medium must first be one in which the solids are dispersiblewith an adequate degree of stability. Secondly the rheologicalproperties of the organic medium must be such that they lend goodapplication properties to the dispersion.

When the dispersion is to be made into a film the organic medium inwhich the ceramic solids and inorganic binder are dispersed consists ofthe polymeric binder, monomer and initiator which are dissolved in avolatile organic solvent and, optionally, other dissolved materials suchas plasticizers, release agents, dispersing agents, stripping agents,antifouling agents and wetting agents.

The solvent component of the organic medium, which may be a mixture ofsolvents, is chosen so as to obtain complete solution therein of thepolymer and to be of sufficiently high volatility to enable the solventto be evaporated from the dispersion by the application of relativelylow levels of heat at atmospheric pressure. In addition, the solventmust boil well below the boiling point and decomposition temperature ofany other additives contained in the organic medium. Thus, solventshaving atmospheric boiling points below 150° C. are used mostfrequently. Such solvents include benzene, acetone, xylene. methanol,ethanol, methylethyl ketone, 1,1,1-trichloroethane, tetrachloroethylene,amyl acetate, 2,2,4-triethyl pentanediol-1,3-monoisobutyrate, toluene,methylene chloride, and ethylene glycol monoalkyl and dialkyl etherssuch as ethylene glycol mono-n-propyl ether, For casting films,methylene chloride is particularly preferred because of its volatility.

Frequently the organic medium will also contain one or more plasticizerswhich serve to lower the T_(g) of the binder polymer. Such plasticizershelp to assure good lamination to ceramic substrates and enhance thedevelopability of unexposed areas of the composition. However, the useof such materials should be minimized in order to reduce the amount oforganic materials which must be removed when the films cast therefromare fired. The choice of plasticizers is, of course, determinedprimarily by the polymer which must be modified. Among the plasticizerswhich have been used in various binder systems are diethyl phthalate,dibutyl phthalate butyl benzyl phthalate dibenzyl phthalate, alkylphosphates, polyalkylene glycols, glycerol, poly(ethylene oxides),hydroxy ethylated alkyl phenol, tricresyl phosphate triethyleneglycoldiacetate and polyester plasticizers. Dibutyl phthalate is frequentlyused in acrylic polymer systems because it can be used effectively inrelatively small concentrations.

The photosensitive compositions of the invention will frequently beemployed as the photosensitive layer of a resist element in which thephotosensitive layer is coated upon a support film.

In conventional photoresist elements, it is necessary, or at leasthighly desirable, to protect the photosensitive layer by a removablecover sheet in order to prevent blocking between the photosensitivelayer and the reverse surface of the support when they are stored inroll form. It is also desirable to protect the layer laminated to asubstrate by means of the removable support film during imaging exposureto prevent blocking between the layer and the phototool.

The photopolymerizable composition is coated upon the support film at adry coating thickness of about 0.001 inch (˜0.0025 cm) to about 0.01inch (˜0.025 cm) or more. A suitable strippable support which preferablyhas a high degree of dimensional stability to temperature changes may bechosen from a wide variety of films composed of high polymers, e.g.,polyamides, polyolefins, polyesters, vinyl polymers, and celluloseesters and may have a thickness of from 0.0005 inch (˜0.0013 cm) to0.008 inch (˜0.02 cm) or more. If exposure is to be made before removingthe strippable support, it must, of course, transmit a substantialfraction of the actinic radiation incident upon it. If the strippablesupport is removed prior to exposure, no such restrictions apply. Aparticularly suitable support is transparent polyethylene terephthalatefilm having a thickness of about 0.001 inch (˜0.0025 cm).

When an element contains no removable, protective cover sheet and is tobe stored in roll form, the reverse side of the strippable supportpreferably has applied thereto a thin release layer of a material suchas wax or silicone to prevent blocking with the photopolymerizablestratum. Alternatively adhesion to the coated photopolymerizable layermay be preferentially increased by flame treating or electricaldischarge treating the support surface to be coated.

Suitable removable, protective cover sheets when used may be chosen fromthe same group of high polymer films described above and may have thesame wide range of thicknesses. A cover sheet of 0.001 inch (˜0.0025 cm)thick polyethylene is especially suitable. Supports and cover sheets asdescribed above provide good protection to the photopolymerizable resistlayer during storage prior to use.

It is preferred that the weight ratio of the inorganic solids(dielectric end glass) to organics be within the range of 2.0 to 6.0and, more preferably, from 2.6 to 4.5. A ratio of no more than 6.0 isnecessary to obtain adequate dispersion and rheological properties.However, below 2.5, the amount of organics which must be burned off isexcessive and the quality of the final layers suffers. The ratio ofinorganic solids to organics is dependent on the particle size of theinorganic solids, the organic components and on surface pretreatment ofthe inorganic solids. When the particles are treated with organosilanecoupling agents, the ratio of inorganic solids to organics can beincreased. It is preferred to use a lower level of organics to minimizefiring defects. It is especially important that the ratio of inorganicsto organics be as high as possible. Organosilanes suitable for use inthe invention are those corresponding to the general formula RSi(OR')₃in which R' is methyl or ethyl and R is selected from alkyl,methacryloxypropyl, polyalkylene oxide or other organic functionalgroups which interact with the organic matrix of the film.

On the other hand, when the dispersion is to be applied as a thick filmpaste, conventional thick film organic media can be used withappropriate rheological adjustments and the use of lower volatilitysolvents.

When the compositions of the invention are formulated as thick filmcompositions, they will usually be applied to a substrate by means ofscreen printing. Therefore, they must have appropriate viscosity so thatthey can be passed through the screen readily. While the rheologicalproperties are of primary importance, the organic medium is preferablyformulated also to give appropriate wettability of the solids and thesubstrate, good drying rate, dried film strength sufficient to withstandrough handling and good firing properties. Satisfactory appearance ofthe fired composition is also important.

In view of all these criteria a wide variety of inert liquids can beused as organic media. The organic medium for most thick filmcompositions is typically a solution of resin in a solvent. The solventusually boils within the range of 130°-350° C.

Especially suitable resins for this purpose are polymethacrylates oflower alcohols and monobutyl ether of ethylene glycol monoacetate withan acid containing moiety.

The most widely used solvents for thick film applications are terpenessuch as alpha- or beta-terpineol or mixtures thereof with other solventssuch as kerosene, dibutylphthalate, butyl carbitol, butyl carbitolacetate, hexamethylene glycol and high boiling alcohols and alcoholesters. Various combinations of these and other solvents are formulatedto obtain the desired viscosity and volatility requirements for eachapplication. In accordance with conventional techniques in the art thefinal composition may be thioxotropic or possess Newtoniancharacteristics dependent on the additives introduced into thecomposition.

The ratio of organic medium to inorganic solids in the dispersions canvary considerably and depends upon the manner in which the dispersion isto be applied and the kind of organic medium used. Normally, to achievegood coverage, the dispersions will contain complementally by weight50-90% solids and 50-10% organic medium. Such dispersions are usually ofsemifluid consistency and are referred to commonly as "pastes".

The pastes are conveniently prepared on a three-roll mill. The viscosityof the pastes is typically within the following ranges when measured.The amount and type of organic medium (vehicle) utilized is determinedmainly by the final desired formulation viscosity and print thickness.

Aqueous Binder

The binder polymer, which is critical for semi-aqueous processabilityand results in high resolution contains a copolymer or interpolymer of aC₁ -C₁₀ alkyl acrylate or C₁ -C₁₀ methacrylate, and an ethylenicallyunsaturated carboxylic acid containing moiety which is at from 5 to lessthan 15% by weight of the total polymer weight. A preferred range isfrom 8 to 12%. Suitable copolymerizable carboxylic acids includeethylenioally unsaturated monocarboxylic acids such as acrylic,methacrylic and crotonic acids and ethylenically unsaturateddicarboxylic acids such as fumaric, itaconic, citraconic, vinyl succinicand maleic acids as well as their half esters and, where appropriate,their anhydrides and mixtures thereof. Because they are cleaner burningin low-oxygen atmospheres, methacrylic polymers are preferred overacrylic polymers.

Within the above-described limits for the nonacidic comonomers, it ispreferred that the alkylacrylate or methacrylate constitute at least 70and preferably 75 wt. % of the polymer.

For the noncarboxylic acid portion polymer binder can contain up toabout 50 wt .% of other nonacrylic and nonacidic comonomers as asubstitute for the alkyl acrylate or methacrylate portion of the polymersuch as derived from styrene, acrylonitrile vinyl acetate, acrylamide,aminoalkylacrylates or methacrylates and the like so long as thepreviously discussed compositional criteria are met as well as thephysical criteria mentioned below. However, it is preferred to use notmore than about 25 wt. % of such monomers because they are moredifficult to burn out cleanly.

Therefore in addition to the above-described acrylic and methacrylicpolymers, various polyolefins such as polyethylene, polypropylene,polybutylene, polyisobutylene, and ethylene-propylene copolymer can alsobe used. Also useful in the invention are the so-called polyethers whichare polymers of lower alkylene oxides, such as polyethylene oxide.

polymer can be made by those skilled in the art of acrylatepolymerization by conventional solution polymerization techniques.Typically, such acidic acrylate polymers are prepared by combining analpha, beta-ethylenically unsaturated acid with one or morecopolymerizable vinyl monomers in a relatively low boiling (75°-150° C.)organic solvent to obtain a 10 to 60% solution of the monomer mixture,then subsequently causing the monomers to polymerize by the addition ofa polymerization catalyst and heating the mixture at the refluxtemperature of the solution at atmospheric pressure. After thepolymerization reaction is essentially complete, the resulting acidpolymer solution is cooled to room temperature and samples are removedto determine the viscosity, molecular weight, acid equivalent, etc. ofthe polymer.

The presence of the acidic monomer component of the composition iscritical to this technology. The acid functionality yieldsdevelopability in a mixture of water and a water-miscible organicsolvent such as a developer containing 0.62 percent sodium borate and8.7 percent butyl cellosolve. Developability herein can be referred to asemi-aqueous processability such as disclosed in Alles U.S. Pat. No.3,458,311 which is incorporated by reference herein.

Additionally it is necessary to keep the molecular weight of the acidcontaining binder-polymer to a value not greater than 100,000,preferably not greater than 50.000 and more preferably not greater than20 000.

The Tg of the binder polymer preferably is above 100° C., as arequirement of the paste is that, after being screen printed the pasteis dried at temperatures up to 100° C. A Tg below this value generallyresults in a very tacky composition. A lower Tg value can be employedfor a material applied other than by screen printing.

Dispersant

A dispersant is used to insure the efficient wetting of the inorganic bythe organic polymers and monomers. A thoroughly dispersed inorganic isdesirable to the preparation of a photoactive paste with the neededcharacteristics of good screen printing and leveling and fire outcharacteristics. The dispersant acts to allow the polymeric binder toassociate or wet the inorganic solids, giving an agglomerate freesystem. The dispersants of choice are the A-B dispersants generallydescribed in "Use of A-B Block polymers as Dispersants for Non-aqueousCoating Systems" by H. L. Jakubauskas, Journal of Coating Technology,Vol. 58; Number 736; pages 71-82. Useful A-B dispersants are disclosedin U.S. Pat. Nos. 3,684,771; 3,788,996; 4,070,388 and 4,032,698 and U.K.Patent No. 1,339,930 each of which are incorporated herein by reference.A preferred class of A-B dispersants are polymeric materials disclosedin U.S. Pat. No. 4,032,698 supra represented by the structure ##STR1##where Q is a polymeric or copolymeric segment of

a. an ester of acrylic acid or methacrylic acid with an alkanol of 1-18carbon atoms:

b. styrene or acrylonitrile:

c. a vinyl ester whose ester moiety contains 2-18 carbon atoms; or

d. a vinyl ether:

X is the residue of a chain transfer agent:

Y is the residue of a di-, tri, or tetraisocyanate radical after removalof isocyanate groups:

A is the residue of a basic radical which, as an entity before reaction,has a pk.sub.α value of 5-14, or a salt thereof; and

m and n are 1, 2 or 3, the total not exceeding 4, provided that when nis 2 or 3, only of A need be as defined.

A particularly preferred member of this class is a polymeric material,hereinafter identified as A-B Dispersant I, represented by the structure##STR2##

where Q is a methylmethacrylate polymeric segment having a weightaverage molecular weight between 6000 and 8000. Also particularlypreferred is a member of a class of polymeric materials represented bythe structure ##STR3## wherein Q is an alkyl methacrylate polymericsegment containing about 20 units of butyl methacrylate, n is 20, m is 8to 12 and R is a chain terminator residue.

Minor amounts of other components can be present in thephotopolymerizable compositions, e.g.. pigments, dyes, thermalpolymerization inhibitors, adhesion promoters, such as organosilanecoupling agents plasticizers coating aids such as polyethylene oxides,etc, so long as the photopolymerizable compositions retain theiressential properties. Organosilanes are particularly useful inquantities of 3.0 wt. % or less based on the weight of the inorganicparticles. Treated particles have a lower demand for organics. Thus, thelevel of organics in the coating can be reduced which results in easierburnout upon firing. The organosilane can also improve the dispersionproperties and allow a lower inorganic binder/ceramic solids ratio atequivalent hermeticity.

PROCESSING

The photosensitive ceramic coating compositions are conventionallyapplied to a substrate in formation of a layer such as in the form of afilm applied to the substrate or in the form of a paste applied such asby silk screening. Thereafter the ceramic composition is imagewiseexposed to actinic radiation to obtain areas which have been exposed toactinic radiation and areas which have not been exposed. Unexposed areasof the layer are removed in a process known as development. For aqueousdevelopment the layer will be removed in portions which are not exposedto radiation but exposed portions will be substantially unaffectedduring development by a liquid such as aqueous solutions containing0.62% sodium borate and 8.7% butyl cellosolve by weight within thetypical development duration. Generally development takes place in a oneto four minute time interval.

Other processing steps which may be conventional can take place before afiring operation takes place, particularly in a substantiallynonoxidizing atmosphere, to volatilize organic components and sinter theinorganic binder and ceramic solids.

In the following examples all parts and percentages are by weight anddegrees are in centigrade unless otherwise indicated.

Inorganics

Glass Frit: Ferro Glass #3467 milled in water, to a particle size ranged50 of 2.3-2.7 microns, magnetically separated, and freeze dried:composition (component mole %); lead oxide (5.6), silicone dioxide(68.1), boron oxide (4.7), alumina (6.5), calcium oxide (11.1), sodiumoxide (2.8) and potassium oxide (.1.3).

Alumina: Aluminum Oxide (Al₂ O₃): particle size d50=2.4-2.6 microns,surface area 3.5-4.5 m² g. Pigment: Cobalt aluminate (CoAl₂ O₄).

POLYMERIC BINDER

Cp 19 Y: Copolymer of 90% methylmethacrylate and 10% methacrylic acid.Mw=50,000, Tg=120° C., Acid No. 120.

Solvent

Butyl Carbitol Acetate

Monomers

TEOTA 1000 - polyoxyethylated trimethylolpropane triacrylate; Mw 1162

Initiator

TBAQ: 2-tert. Butylanthraquinone

Stabilizer

Antioxident: Ionol:2,6-di-tert.-butyl-4-methylphenol. Dispersant

A-B Dispersant I - see description above

PREPARATION OF DIELECTRIC PASTE A. Preparation of Organic Vehicle

Organic components, solvent and acrylic polymer are mixed and heatedwith stirring to 135° C. and heating and stirring continued until allthe acrylic polymer has dissolved. The solution is then cooled to 100°C. and the initiator and stabilizer added. This mixture is then stirredat 100° C. until the solids have dissolved, after which the solution ispassed through a 400 mesh filter and allowed to cool.

B. preparation of Dielectric Inorganics--Glass Frit

The glass frit, Ferro 3467 8 Kg, is milled in 8 liters of water in aSweco Mill using 0.5 in diameter by 0.5 in long alumina cylinders forapproximately 16 hours to achieve a d50 particle size distribution of2.3-2.7 microns. The frit water mixture is then passed through a 400mesh screen and passed through a S. G. Franz Model 241F2 MagneticSeparator at a DC setting of 11.5 V and 30 amps.

The glass frit mixture is then freeze dried using a Virtis Consol 12Freeze Drier. This procedure usually requires three days to remove allthe water.

Ceramic--Alumina

The alumina A14, as received from Alcoa Inc., 8 Kg is milled in 8 litersof water for from 14-16 hours in a Sweko Mill using 0.5 in diameter by0.5 in long alumina cylinders. This milling time and configuration givesa d50 particle size range of 2.3-2.7 microns. The water alumina mixtureis then passed through a 400 mesh screen and passed through a S. G.Franz Model 241F2 Magnetic Separator at a D.C. setting of 11.5 V and 30Amps. The water alumina mixture is freeze dried using a Virtis Consol 12Freeze Drier. This procedure requires approximately three days to removeall the water. The resulting alumina powder is an agglomerate free freeflowing powder.

Alumina--Cobalt Aluminate

The alumina 8 Kg and cobalt aluminate pigment 144 gm are mixed in 8liters of water and milled in the Sweco using 0.5 in diameter by 0.5 inlong alumina cylinders for one hour. This is done to thoroughly mixthese materials. The milled alumina-pigment mixture is passed through a400 mesh screen and then through a S. G. Franz Midel 241F2 MagneticSeparator at a D. C. Setting of 11.5 V and 30 Amps. The mixture isfreeze dried in a Virtis Consol 12 Freeze Drier. This requiresapproximately three days to remove all the water.

C. PASTE FORMULATION

The dielectric paste is prepared, under yellow light, by mixing theorganic vehicle, monomer, and dispersant into a mixing vessel. The glassfrit and alumina/cobalt aluminate mixture is then added. The compositionis then mixed for 30 minutes. The mixture is aged for approximately 12hours and then roll milled using a three roll mill, at a roll pressureof 400 psi. Usually five passes through the mill are sufficient tothoroughly mix the composition. The paste is then screened through a 400mesh screen.

The paste viscosity at this point is adjusted by the addition of butylcarbitol acetate to 80-120 P.S. This viscosity range is optimum forscreen printing.

D. PROCESS CONDITIONS

Care is taken to avoid dirt contamination in the process of preparingcoating compositions and in preparing dielectric parts since suchcontamination can lead to defects in the fired dielectric. The processwork is best done in a class-100 clean room.

The paste is applied to ceramic parts by screen printing using a 325mesh patterned screen. The parts are dried at between 75°-100° C. in anitrogen or air atmosphere oven. The dried coating has a thickness of 20microns. A second printing and drying step is normally performed to givea thicker part (40 microns).

The parts are exposed, by contact with the phototarget, with either aBerkey-Askor vacuum printer or a columnated HTG UV exposure source,using a 15 second nitrogen purge and a 15 second drawdown in the vacuumprinter. The optimum exposure time is determined from an exposure seriesthat yields information on the best exposure to yield the correct sizevias or photoformed holes in the dielectric after development.

The exposed parts are developed using a Du Pont ADS-24 processorcontaining 0.62 percent sodium borate. 8.7% butyl cellosolve and theremainder water. Temperature can be varied from 20°-45° C. Solvent issprayed at 30 psi for a development rate of 3.4-15 ft/minute through a 4ft chamber.

The developed parts are dried in a forced draft oven at 75° C. for 15min and fired in a furnace with peak temperature of 900° C. over atwo-hour cycle. In firing the composition of the invention, is exposedto a substantially nonoxidizing atmosphere up to the glass transitiontemperature of the inorganic binder and to an essentially completelynonoxidizing atmosphere during the sintering phase of the final step.

By the term "substantially nonoxidizing atmosphere" is meant anatmosphere which contains insufficient oxygen to effect any significantoxidation of copper metal, but which nevertheless contains sufficientoxygen to effect oxidation of the organic materials. In practice, it hasbeen found that a nitrogen atmosphere of 100-1000 ppm O₂ is appropriatein the presintering phase of the firing step. From 300 to 800 ppm O₂ ispreferred. The amount of oxygen is increased as the thickness of thedielectric layer increases. For two layers of dielectric paste whichfire out to 20 microns. 300 to 400 ppm O₂ is sufficient. On the otherhand, the essentially completely nonoxidizing atmosphere used during theglass sintering step of the firing step refers to a nitrogen atmospherecontaining only residual amounts of O₂, e.g., about 10 ppm. It ispreferred to fire the composition of the invention at low heating ratesin order to minimize physical defects in the fired layer.

In each of the following examples the following was employed;

    ______________________________________                                        Alumina/Cobalt Aluminate                                                                           %                                                        Alumina              98.2                                                     Cobalt Aluminate     1.8                                                      Glass Frit           %                                                        SiO.sub.2            40.2                                                     BaCO.sub.3           17.9                                                     Al.sub.2 O.sub.3     9.9                                                      ZnO                  8                                                        MgO                  5                                                        B.sub.2 O.sub.3      5.9                                                      CaO                  5.1                                                      PbO                  8                                                        ______________________________________                                    

Test Procedures

A capacitor is formed from the above-described dielectric filmcomprising a copper disk having an area of 1 cm² and a contact tabsupported on an alumina substrate. Overlying the copper disk is a layerof the dielectric film and overlying the dielectric layer is a secondcopper disk of the same size having a contact tab rotated 90°-180° withrespect to the lower tab.

Capacitance and dissipation factors are measured at 1 kHz using aHewlett-packard Hp4274A multi-frequency LCR meter, while insulationresistance is measured using a Super megohm meter Model RM 170 (BiddleInstruments, AVO, Ltd., U.K.). Insulation resistance measurements aremade after charging the capacitor to 100 VDC. Each number is the averageof at least 10 measurements. The thickness of the dielectric layer ismeasured using a Gould Surfanalyzer 150/recorder 250. The dielectricconstant is calculated using the equation: ##EQU1##

C is the capacitance of the capacitor;

A is the area of small electrode in contact with the dielectric layer;and t is the thickness of the dielectric layer.

All capacitors were aged for at least 15 hours after firing beforemaking the electrical measurements. It is common that the dissipationfactor (DF) decreases by 0.5-2% within this aging time period. However,capacitance is generally unaffected during this period.

Dissipation factor on a wet basis is determined by placing a drop ofwater on the upper copper disk so that it wets the disk but not thecontact tab. After standing for 30 seconds DF is determined in the usualmanner. The dielectric is considered to be hermetic when the wet DF isless than 1%. Below 0.5% is preferred.

                  Example 1                                                       ______________________________________                                        Component               Parts                                                 ______________________________________                                        Alumina/Cobalt Aluminate                                                                              27.6                                                  Glass Frit              27.4                                                  Vehicle                 31.0                                                  Binder: CP-19Y       36.0                                                     Solvent: Butylcarbitolacetate                                                                      54.3                                                     Initiator: TBAQ      5.4                                                      Antioxident: Ionol   0.3                                                      Monomer: TEOTA          8.4                                                   Dispersant: A-B Dispersant I                                                                       2.1                                                      Results                                                                       Resolution:    6-8 mil vias                                                   Photo Speed:   50-90 mj/cm.sup.2                                              Development:   0.62% Sodium Borate, 8.7%                                                     Butyl Cellosolve (2-butoxyethanol)                                            in water.                                                      ______________________________________                                    

                  Example 2                                                       ______________________________________                                        Component               Parts                                                 ______________________________________                                        Alumina/Cobalt Aluminate                                                                              27.6                                                  Glass Frit              27.4                                                  Vehicle                 31.0                                                  Binder: CP-19Y       35.0                                                     Solvent: Butylcarbitolacetate                                                                      59.9                                                     Initiators:          4.80                                                            Benzophenone  4.0                                                             Michler's ketone                                                                            0.80                                                     Antioxident: Ionol      0.30                                                  Monomer: TEOTA          10.6                                                  Dispersant: A-B Dispersant I                                                                          2.1                                                   Results                                                                       Resolution:    6-8 mil vias                                                   Photo Speed:   50-90 mj/cm.sup.2                                              Development:   0.62% Sodium Borate, 8.7% Butyl                                               Cellosolve (2-butoxyethanol) in                                               water                                                          ______________________________________                                    

What is claimed is:
 1. In a method of making a photosensitive ceramiccoating composition which is fireable in a substantially nonoxidizingatmosphere comprising an admixture of:(a) finely divided particles ofceramic solids having a surface area-to-weight ratio of no more than 10m² /g and at least 80 wt. % of the particles having a size of 1-10 μm,and (b) finely divided particles of an inorganic binder having a glasstransition temperature in the range from of 550° to 825° C., a surfacearea-to-weight ratio of no more than 10 m² /g and at least 90 wt. % ofthe particles having a size of 1-10 μm, the weight ratio of (b) to (a)being in a range from 0.6 to 2, dispersed in an organic mediumcomprising (c) an organic polymeric binder, and (d) a photoinitiationsystem, dissolved in (e) photohardenable monomer, and (f) an organicmediumwherein the improvement comprises an organic polymeric bindercontaining a copolymer or interpolymer of a C₁ -C₁₀ arkyl acylate or C₁-C₁₀ alkyl methacrylate, or combinations thereof and an ethylenicallyunsaturated carboxylic acid, wherein a moiety in the binder derived fromthe unsaturated carboxylic acid comprises from 5 to less than 15 weightpercent of the polymer and wherein the binder has a molecular weight notgreater than 100,000 and wherein the composition upon imagewise exposureto actinic radiation is developable in an aqueous solution containing byweight 0.62 percent sodium borate and 8.7 percent butyl cellosolvewherein prior to mixing of component (a), (b), (c), (d), (e), (f),component (a) of finely divided particles of ceramic solids andcomponent (b) of finely divided particles of an inorganic binder arefreeze dried in formation of such particles.
 2. The method of claim 1wherein component (a) of finely divided particles of ceramic solids arepassed through a magnetic separator.
 3. The method of claim 2 whereinthe organic medium contains a dispersant.